Current safety restraint systems have a seat belt which is mechanically retracted against the shoulder of the occupant of the seat. In many applications, the tension of the shoulder portion of the seat belt against the shoulder, and often against the neck of the occupant, has been a source of irritation. For the seat belt to be fully retracted onto the spool of the retractor in the undeployed state, a sufficient force must be generated by the retractor's retraction spring to rotate the retractor's spool a required number of turns. As a consequence, when the safety restraint system is deployed, the shoulder portion of the seat belt exerts excess force on the shoulder of the occupant.
To eliminate this problem, some of the early models of seat belt retractors had a manually adjustable tension remover in which a portion of the seat belt could be unwound from the retractor's spool and the retractor's spool would automatically be prohibited from rewinding the manually adjusted slack. However, this proved to be unsatisfactory from a safety point of view because many of the occupants adjusted the seat belt to have too much slack. This resulted in the occupant's upper body, and in particular, the occupant's head, to be thrown against the dashboard of the vehicle or the windshield in the event of an accident or sudden stop.
Frantom et al., in U.S. Pat. No. 4,655,312, teaches an electrically adjustable safety restraint system in which an electric motor is energized in response to the buckling of the adjustable tongue into buckle of the safety restraint system to wind the seat belt on the retractor's spool until a predetermined tension is detected. After the predetermined tension is sensed, the electric motor is reversed for a predetermined period of time selected to produce a predetermined slack on the shoulder portion of the seat belt.